Electrical control circuit



' Sept. 15, 1959 L. L. LITTLE 2,904,736

ELECTRICAL CONTROL qmcurn Filed May 6, 1957 United States Patent ELECTRICAL coNrr-ior. CIRCUIT Larry L. Little, Los Angeles, Calif, assignor to Western Precipitation Corporation, Los Angeles, Calif., a corporation of California Application May 6, 1957, Serial No. 657,288

12 Claims. (Cl. 318130) The present invention relates generally to electrical control circuits and more particularly to control circuits of the type which are used to energize periodically electromagnetic hammers or similar pieces of equipment. However, it will be realized that in its broad aspect, my invention is not necessarily limited to any specific type or use of the equipment which is energized.

In the field of dust collecting equipment, electrical precipitators are well known as a means of collecting dust particles which are suspended in a gas stream. As a result of the action of an electrostatic field upon the particles after they have been electrically charged, these particles are deposited upon a collecting electrode, which is usually a member of extended surface area, as for example a plate. The dust particles when once deposited upon the surface of the collecting electrode, generally tend to adhere to the surface. As a consequence, it is common practice to remove the particles from the collecting electrode by vibrating the electrode to jar the dust particles loose, after which the dust particles fall freely under the influence of gravity into a suitable hopper.

Many different means have been devised for rapping or vibrating the collecting electrodes. Various types of mechanical means have been tried but for various reasons it is advantageous to apply the blows to the collecting electrodes by means of electro-magnetic hammers, usually referred to in the art as rappers. Briefly, each rapper consists of a large coil through which an electric current of short duration is passed at controlled intervals in order to establish a magnetic field which drives the hammer against an anvil mounted on or connected to the electrodes, a portion at least of the hammer forming a movable core within the coil.

in an installation handling a large volume of gas, the electrical precipitator normally has a large number of collecting electrodes requiring a substantial number of rappers, typically in the range of 20 to 50. For various technical and economic reasons which need not be recited in detail here, it is often desirable to energize several of the rappers at the same time, these several rappers being connected as a group in parallel with each other. There are ordinarily several such groups of rappers, the groups of rappers being energized successively.

Thus it becomes a general object of my invention to provide a control circuit adapted to energize simultaneously a plurality of electric rappers, the number of rappers not being limited by limitations inherent in the circuit on the amount of energy that can be made available instantaneously. This is in contrast with energizing circuits previously known which rely upon a large capacitor to store electrical energy and which ordinarily are adequate only to energize one or sometimes two rappers. It will be realized that the capacitor, unless made of an impractically large size, places a definite and undesirably low limit upon the amount of electrical energy which may be supplied to a group of rappers to energize them in any given instance; and it has been found preferable to dispense with this capacitor and its limitation and instead to connect directly to a source which may be considered as substantially unlimited in its energy output, for practical purposes.

In ordinary industrial service, electrical precipitators are in continuous service so that if the time interval between successive energizations of rappers, either singly or in groups, is one minute, over 50,000 switch operations are required within a period of only one year to effect or regulate successive energizations. Of course, the number of switchings increases as the interval between them is shortened; and a typical installation energizing rappers at the rate of ten times per minute requires over 500,000 switchings per year. It is obvious that under this type of duty, mechanical switches controlling the pulses will soon wear out and fail.

Consequently, it is also an object of the invention to provide an electrical control circuit in which mechanical switch means for controlling the energizing operations are unnecessary and switching operations for this purpose are carried out with static-type components, completely eliminating the necessity of mechanically moving parts in the control circuit.

Because of the fact that the electrical precipitators are in continuous duty and are normally part of processes which can not be shut down or discontinued except at considerable expense, great care must be taken that the control circuit is entirely dependable and reliable in operation. In addition to providing these characteristics, it is also an object of my invention to provide a control circuit which is simple and economical in construction.

It is a further object of the invention to incorporate in a control circuit of the character described, means for adjusting both the frequency and the intensity of the blows struck by the rappers in order that their operation may be easily adjusted to the conditions under which the dust is collected in the precipitator.

The above objects and advantages of my invention are attained in an electric circuit for periodically energizing an electro-magnetic hammer or the like, that is characterized by utilizing directly as a power source alternating current which is ordinarily supplied commercially. This power source does not place any limitations, inherent in the control circuit, upon the quantity of energy that can be withdrawn to energize the rappers. Electronic switch means is provided, connecting the power source to the rappers, for periodically delivering an electric power pulse to the rappers. This switch means is controlled by switch operating means comprising a positive feedback circuit energized in response to current flow from the power source and connected to the switch means to place the switch means periodically in conductive condition; and comprising also a negative feedback circuit energize in response to the output voltage of the switch means to restore the electronic switch means to a non-conducting condition.

How the above and other objects of my invention, as Well as others not specifically mentioned, are attained, will be more readily understood by reference to the annexed drawing, the single figure of which is a schematic diagram of a presently preferred form of my invention.

Referring now to the drawing, the leads 10 and 1 are connected to a suitable source of alternating current. This source may be any convenient available source, typically a commercial power line at 440 volts and 60 cycles. However, the invention is not necessarily limited to a power source having these'particular characteristics. Conductor 10 is connected to one side of the primary coil of the step-up power transformer PT, the other side of the primary coil being connected by conductor 12 to one side of the primary coil of current transformer CTl. The other side of the primary coil of the latter transformer is connected to conductor 11, so that the primaries of the two transformers are connected in series and the same current flows through both.

Transformer PT is of the center tap type, the center point of the secondary of the transformer being connected by conductor 14 to the load, as will be described. The opposite ends of the secondary of transformer PT are connected one each to the plate of a thyratron tube 15. Tubes 15 are gas-filled triodes each having a grid that controls the firing potential. The tubes 15 are here employed as grid-controlled rectifiers; and two tubes are employed in order to obtain full wave rectification of the gutput from the secondary of the power transformer The filaments of the two tubes 15 are connected in parallel to the secondary of filament transformer CT2 by conductors 16 and 17. The filament transformer GT2 is likewise of the center-tap type and the center of the secondary coil is connected by conductor 18 to the load, as will be further described. The primary coil of the filament transformer is connected to conductors 19 and 20 which are in turn connected to a suitable source of electric power. As typical of such a power source, conductors 19 and 20 are here indicated as being connected to a source of commercial alternating current at a potential of 110 volts and a frequency of 60 cycles.

The portion of the control circuit so far described serves as a source of electric pulses which are supplied to the load, in this case the electric rappers in the precipitator. This pulse source consists generally of a transformer connected to a power source and electronic switch means which turns on and off the electric pulses that are supplied to the load. Tubes 15 serve not only as rectifiers but also as the electronic switch. This electronic switch is operated by means constituting a part of the control circuit.

The two conductors 14 and 18 supply current to the electrical rappers which are indicated only diagrammatically in the drawing by the coils 21, since each rapper has one such coil which is energized in order to cause the rapper to strike a blow against a collecting electrode. The present invention permits the rappers to be energized in groups, with all of the coils 21 in any one group arranged in parallel. All of the coils are connected to a common return, as for example the conductor 14. A plurality of coils, shown on the drawing as three, but which number may be more or less, are connected in parallel to one conductor 18a which is a branch of conductor 18. A second group of coils, which may be the same in number or more or less than the first group, is connected in parallel between conductor 14 and a second branch conductor 18b. Additional groups of coils may be likewise connected between conductor 14 and other branch conductors 18c, 18d, etc. Each one of the branch conductors 18a through 18 is connected to a separate tap of pulse distribution switch SW1, which may be a stepping switch or other similar mechanical switching mechanism which can connect the branch conductors in succession to the main conductor 18 here shown as being connected to the rotating arm 22 of the stepping switch. Rotating arm 22 is driven by any suitbale means; as for example by solenoid 23, which is supplied with power by conductors 19 and 20. Each time SW2, in series with solenoid 23, is closed, solenoid 23 is energized and it causes arm 22 to advance from one tap to the next. Of course, SW2 may be operated manually, but preferably is operated by a suitable switch operating mechanism (not shown) that closes it at desired intervals of time. Although not shown, as it is not part of the present invention, means may be provided to prevent SW2 from being closed during delivery of a pulse from tubes 15.

The electronic current switchtubes 15-is turned on and off by switch operating means which may be considered as divided into two separate portions. The first portion is a current feedback loop connected to the power source through transformer CTI, and which op- 2,9o4,7se t I erates to render the electronic switch conductive to turn the current on for the rappers. The other or second portion of the switch operating means is a voltage feedback loop connected to the output side of the electronic switch. It operates to render the tubes non-conductive; and thereby turns oif the power for the rappers.

Being connected to conductors 11 and 12, the primary side of current transformer CT1 receives the same current that passes through the primary of power transformer PT. The secondary of the transformer CT1 is connected across opposite sides of a full-wave rectifier bridge 24 of a suitable design. The output terminals of the rectifier bridge are connected respectively to conductors 26 and 25. Capacitor C1 is connected across the output terminals of the rectifier bridge, being connected to conductors 26 and 25. A fixed resistance R1 is shunt-connected across capacitor C1.

In series with each other and with conductor 26, are choke coil L1 and capacitor C2. The other terminal of. capacitor C2, indicated at 27, is connected by conductors 28 and 29 to the grids of the two tubes 15. Current limiting resistors R4 and R5 are respectively each connected in series with one of the two grids and conductor 28 or 29. As will be described later in more detail, the current feedback circuit operates to turn the current on by a predetermined change in the voltage at terminal 27, which is the voltage applied to the tube grids by conductors 28 and 29. Current is turned on by causing the potential at 27 to approach zero from a high negative value, with respect to the conductor 25, due to the discharge of capacitor C3 through resistors R2 and R3.

The voltage feedback circuit operates to turn the power off after a pulse is supplied to the rappers 21 in order to permit switching from one rapper, or group of rappers, to another, and also to produce successive strokes of the same rapper. This negative feedback loop of the switch operating circuit is conductively connected to conductors 14 and 18 which constitute power output leads from the electronic switch and power input leads to the rappers. Conductor 14 is connected by conductor 30 through the series connected resistors R6, R7 and R8 and half-wave rectifier 32 to terminal 27 of capacitor C2. Conductor 18 is connected directly to conductor 25. Terminal 27 is connected through fixed resistor R2 and variable resistor R3 to conductor 25 while storage capacitor C3 is connected between said terminal 27 and conductor 25 in parallel with the two resistors R2 and R3. At a point between R7 and R8, capacitor C4 is connected between conductor 3% and conductor 25, while at some point between half-wave rectifier 32 and conductor 14, conductors 25 and 30 are also connected by capacitor C5 and resistance R9 in series with each other. Conductors 14 and 18 are also connected through half-wave rectifier 34.

In describing the operation of my improved form of control circuit, it is convenient to assume initially that the circuit is in a relatively static condition in which the tubes 15 are non-conducting. Under these circumstances, there is no power flowing to the rappers 21 because the electronic switch is oif. The potential of terminal '27 is now highly negative with respect to conductor 25, this voltage being impressed through conductors 28 and 29 upon the grids of both tubes 15. This negative voltage is present because of a charge in capacitor C3 which is sufficient to prevent the tubes from being conductive. Resistors R2 and R3 are in series with each other and are connected across the terminals of capacitor C3, so these resistors comprise a circuit which permits the charge on capacitor C3 to bleed off at a predetermined rate. As the result of this loss of charge, terminal 27 becomes less negative with the passage of time and its potential shifts toward a zero value. Eventually the potential at the control terminal 27 approaches sufliciently close to zero that one of the tubes 15 becomes conductive for at least one-fourth cycle and current flows through that tube. It makes no difference which tube.

A flow of current through one of the tubes immediately causes a marked increase in current flowing in the primary of the power transformer. There is a like increase in current fiowing in the primary of current transformer CT1. In response to this current flowing in the transformer primary, a voltage appears across the input terminals of rectifier bridge 24. As a result of this voltage, and in proportion to it, a current flows from the output terminals of the bridge through conductors 25 and 26. Bridge 24 provides full wave rectification of the current from transformer CT1 and the output from the bridge is a pulsating direct current. Choke coil L1 blocks the ripple in the pulsating current so that a comparatively steady direct current voltage is applied to blocking capacitor C2. Capacitor C1 by-passes the pulsating current, as does the resistor R1 and voltage regulator diode 35. Resistor R1 is provided also to act as a direct current load across the output of bridge 24 so that the bridge is not operating on open circuit and to provide a discharge path for C2. The purpose of regulator diode 35 is to cause a constant voltage to be impressed on capacitor C2 regardless of the intensity of the rapper blow and the number of rappers being energized at one time.

Capacitor C2 blocks the steady direct current which fiows through coil L1 so that this steady-state current does not reach terminal 27; but C2 passes current in direct proportion to the voltage change across the capacitor. This current charges capacitor C3 and so has an effect on the potential of terminal 27. The characteristics of the circuit are such that output current from bridge 24 charges C3 in a manner to render terminal 27 positive. Thus, as a result of current flowing in the primaries of transformers CT1 and PT when the first tube fires, the output current from the bridge charges capacitor C3 in a direction which accelerates making the potential of the control point 27, and likewise the potential of the two grids of the tubes 15, such that both tubes are immediately rendered fully conductive. When this occurs, the electronic switch is turned fully to the on condition and current flows through one of the tubes during each half cycle. The two tubes act as a full wave rectifier and transmit a heavy flow of current from the power source, as represented by leads and 11, through transformer PT and conductors '14 and 18 to one of the groups of rappers 21, depending upon which one of the branch conductors 18a-18f switch arm 22 is connected to at that moment.

This flow of current to the rappers during the on period, even though not uniform, is herein termed a pulse since it continues for only a short time, perhaps .1 to .2 seconds or about 6-12 cycles. During this on" period, the source of power is connected through the electronic switch directly to the rappers 21, and the full power that the source can supply may be drawn upon. No capacitor or other storage device in the circuit supplies power to the rappers and thus limits the power available. As a result, not only one or possibly two but six or eight hammers can be simultaneously energized without any objectionable decrease in the intensity of the blow struck by them.

The electronic current switch is turned to off condition to terminate a pulse period. This is accomplished by the negative feedback portion of the circuit which is connected to the output from the electronic switch to be energized therefrom. Conductors 30 and 25 are connected to conductors 14 and 18, respectively. Hence, the voltage between the last two conductors creates a voltage between conductors 25 and 30.

This voltage charges capacitors C3, C4 and C5, all connected between the two conductors and in parallel with each other. Capacitor C5 with resistance R9 in series is connected across conductors 25 and 30 ahead of rectifier 32 (i.e. between rectifier 32 and conductor 14) and other elements of the circuit to provide a low impedance load for tubes 15. This offsets the high inductance of coils 21 and facilitates placing both tubes 15 in a fully conductive condition immediately that one tube fires. Capacitors C4 and C3 are both separated from C5 by rectifier 32 and resistance R8 and are separated from each other by the resistance provided by R6 and R7. The presence of resistance at R8 and at R6 and R7 causes capacitor C5 to charge first, and as a result followed by C4; there is a delay in charging C3. The greater the total resistance at R6 and R7, the longer the delay; and to permit varying this delay, the total resistance is divided into a fixed portion R6 and a variable portion R7. How ever, it will be realized that the total resistance can be all in a single resistance unit, either fixed or variable.

The characteristics of the circuit are such that when capacitor C3 is charged from the voltage feedback circuit, the potential of terminal 27 becomes negative. This negative potential is opposite to and neutralizes the positive potential at this terminal produced by charging the capacitor by the positive current feedback, as described above. The potential at 27 is applied to the tube grids; and when the grids become sufiiciently negative, current ceases to flow through the tubes. This result occurs because the thyratron tubes deionize in a few microseconds and become nonconductive when the grid voltage is sufficiently negative and the plate voltage drops to zero for a time in excess of the deionifiation time, as it does at the end of each current cycle. By returning the tubes to a non-conductive condition, the electronic switch turns off the current to the rappers and the on period or pulse is terminated. The length of time required to charge condenser C4 and subsequently neutralize and recharge condenser C3 negatively determines the length of the pulse. The presence of this delay characteristic in the voltage feedback circuit, as opposed to the immediate response of the current feedback circuit that turns on the switch, permits the current switch to stay on for a predetermined length of time to deliver a power pulse to the rappers.

Rectifier 32 is provided to prevent the discharge of capacitors C3 and C4 through the path R8 and R9 when the current ceases to flow through conductors 14 and 18.

Rectifier 34 is connected between conductors '14 and 18' to provide a by-pass for current during the decay of the magnetic fields around coils 21 after a pulse period. During the decay of the fields, rectifier 34 passes the continuing current result from the inductance of coils 21.

The charge now on capacitor C3 again bleeds off through the circuit composed of the resistors R2 and R3. The discharge of the capacitor reduces the negative potential on terminal 27 toward zero, allowing the above described cycle of operations to start over again as the grids controlling tubes 15 place the electronic switch inconductive or on condition. Capacitor C3 discharges at a rate determined, at least in part, by the total resistance which R2 and R3 provide. An increase in this resistance increases the time required for discharge of the capacitor and reduces the frequency of blows by the rappers as it increases the interval between on periods. Resistors R2 and R3 are preferably one fixed and one variable in order to permit easy adjustment of the frequency of rapper blows by adjusting the amount of resistance in the circuit. However, the plurality of resistors can be replaced by a single resistance, either fixed or variable.

I claim:

1. In an electrical control circuit for periodically energizing an electro-magnetic hammer or the like from a power source, the combination comprising: electronic switch means periodically electrically connecting the power source to the hammer; and switch operating means comprising a positive feedback circuit electrically connected to the switch means to place the switch means periodically in conductive condition and energized in response to load current fiow from the power source after the conductive condition is initiated to maximize said conductive condition, and a negative feedback circuit 7 energized in response to the output voltage of the switch means and electrically connected to the switch means to return the switch means to a non-conductive condition.

2. An electrical control circuit as in claim 1 in which the positive feedback is immediately responsive to current flow from the power source and the negative feedback has a delayed response to the output voltage from the switch means.

3. An electrical control circuit as in claim 2 in which the negative feedback circuit includes means for varying the delay in response to the output voltage.

4. An electrical control circuit as in claim 2 in which the negative feedback circuit includes adjustable means for varying the intensity of blow delivered by the hammer.

5. In an electrical control circuit for periodically energizing an electro-inagnetic hammer or the like directly from a power source providing alternating current, the combination comprising: combined rectifying and electronic switch means adapted to deliver periodically from the power source a direct current pulse to the hammer; and switch operating means comprising a positive feedback circuit electrically connected to the switch means to place the switch means periodically in conductive condition and energized in response to load current flow from the power source after the conductive condition is initiated to maximize said conductive condition, and a negative feedback circuit energized in response to the output voltage of the switch means and electrically connected to the switch means to return the switch means to a nonconductive condition.

6. In an electrical control circuit for periodically energizing an electro-magnetic hammer or the like from a power source providing alternating current, the combination comprising: a pulse source adapted to deliver periodic pulses of current from the power source to the hammer, including a step-up, center-tap transformer and a pair of gas-filled, grid-controlled rectifying tubes operating in parallel; and switch operating means comprising a positive feedback circuit electrically connected to the switch means to place the switch means periodically in conductive condition and energized in response to load current flow from the power source after the conductive condition is initiated to maximize said conductive condition, and a negative feedback circuit energized in response to the output voltage of the switch means and electrically connected to the switch means to return .the switch means to a non-conductive condition.

7. In an electrical control circuit for periodically energizing an electro-magnetic hammer or the like from a power source providing alternating current, the combination comprising: a pulse source adapted to deliver periodic pulses of current from the power source to the hammer, including a step-up, center-tap transformer and a pair of gas-filled, grid-controlled rectifying tubes operating in parallel; a current feedback circuit connected to the primary of said transformer and including means periodically changing the bias on the grids of both tubes to render at least one tube partially conductive, said feed back circuit being responsive to current thereupon supplied to said step-up transformer to change the bias on the grids .of said tubes in a manner to render both tubes fully conductive; and a voltage feedback circuit connected to the output from said tubes and responsive to output voltage of the pulse to return the tubes to a non-conductive condition.

8 In an electrical control circuit for periodically energizing an .electro-magnetic hammer or the like from a power source providing alternating current, the combination comprising: a pulse source adapted to deliver periodic pulses of current from the power source to the hammer, including a step-up, center-tap transformer and a pair of gas-filled, grid-controlled rectifying tubes operating in parallel; a storage capacitor having one terminal connected to the grids of the tubes to bias the grids; means providing a discharge path for the capacitor to discharge the capacitor at a predetermined rate; means responsive to the input current flow to said transformer to charge said capacitor in a manner to decrease negative polarity at said terminal and at the tube grids; and means responsive to the output voltage from said tubes to charge said capacitor in a manner to increase negative polarity at said terminal and at the tube grids.

9. An electrical control circuit as in claim 8 that also includes adjustable means controlling the rate of discharge of the capacitor.

10. An electrical control circuit as in claim 8 that also includes adjustable means controlling the rate at which the capacitor is charged to produce negative polarity at said capacitor terminal.

11. An electrical control circuit as in claim 10 in which the adjustable means includes a variable resistance be- References Cited in the file of this patent UNITED STATES PATENTS Lindsay May 22, 1945 Taylor June 29, 1948 

